This invention relates to improvements in flow-through filtration type devices for separating immiscible liquids which may carry particulate solids, such as for separating dirty oil from water. A main object of the invention is to provide such a separator apparatus useful in remote oil fields wherein most of the oil of whatever viscosity or density presented (together with iron sulfide particles or other dirt) must be separated from the large volumes of production water coming up with the oil before the water can be lawfully discharged into the open environment. The invention is herein illustratively described by reference to the presently preferred embodiment thereof; however, it will be recognized that certain modifications and changes therein with respect to details may be made without departing from the essential features involved.
Separation of oil (clean or dirty) from water for purposes of environmental protection and for other applications imposes limitations of a practical and economic nature on the types and requirements of separator apparatus that can be used. In many cases, very large volumes of mixture must be processed on a continuing basis. Electrical power to drive mechanical separators or high-pressure pumps is often not conveniently available nor are other forms of high-pressure fluid sources. In some situations, the difference between being able to operate and not operate at all, such as in remote oil fields, can be made by whether sufficient high-volume processing capability exists that will serve reliably over long periods with little or no human attention.
A chief technical problem in achieving the required high degree of separation, such as 99% removal of oil and solid materials from waste water, is occasioned in large part by the emulsified state of the oil, i.e., by the extremely small size and dispersion of oil particles often present in oil well production water. Indeed this is also often true in many industrial applications wherein similar oil/water separators are necessary. These oil particles quite often range in size down to ten microns or less in diameter, making it virtually essential to coalesce the droplets into larger drops and globules before effective separation can take place. To effect such coalescence and separation in a short time period, in a reasonably compact apparatus enclosure, and by arrangements capable of processing at very high volumetric rates under the limitations mentioned above demanded in many installations, including remote oil field production operations, is a broad object of this invention, including in connection therewith also the effective removal of solid particles and the adaptability of the apparatus to work efficiently with oil of high density and viscosity as well as the grades or types lighter than water.
A number of known types of separators are ruled out immediately as unsuitable for the described task. For example, vertical tube coalescers and parallel plate coalescers are not effective with oil droplet sizes below about fifty microns and they provide very limited recovery of oil-wetted solids. Neutrally byoyant oil-wetted solids as well as heavy oils (i.e., having a specific gravity near that of water) pass through such separators virtually undiminished.
Cartridge filter type coalescers are also unsuitable for many such applications. While they remove oil droplets down to about ten microns in diameter and collect oil-wetted solids, they plug too rapidly because of the extremely small pore sizes used in order to be practical for the kinds of jobs indicated herein. One recent study, for example, indicated an average operating life of a cartridge type coalescer of less than about two hours before it was necessary to replace the cartridge in such a conventional task as ship's bilge oil/water separation.
Furthermore, effective operation of a cartridge type separator depends upon establishing a high-pressure drop across the cartridge, i.e., up to 70 pounds per square inch. This requires a high-pressure source, usually a local pump that consumes energy and that itself tends to aggravate the problem of emulsification because of the action of a high-pressure pump in shearing the oil droplets into yet smaller droplets in feeding the separator.
Deep bed media filter coalescers have also been used with effectiveness in some situations but in general are unsuitable for the type of application to which this invention is primarily directed. Such coalescers utilize a bed of granular media, such as sand, glass beads, polypropylene beads or chips, anthracite granules, etc., the bed being usually about nine inches or more in depth. Oil-wetted solids are captured and recovery of oil droplets down to extremely small sizes can be achieved. However, in a deep bed coalescer relatively high pressure drops across the bed are necessary, i.e., not uncommonly as high as fifty pounds per square inch. This usually requires pumping with attendant expenditure of energy and additional emulsification of the oil by the feed pump itself. Furthermore, reasonably uniform regeneration of the bed by backwashing in order to remove accumulated solids and oils is difficult to accomplish. There is a tendancy for the bed to "mud ball" and consequently for the flow to channelize through some areas of the bed in preference to others so that uniform fluidization and regeneration of the granular bed during backwashing is not achieved, especially when the system has been used to separate highly viscous oils. Furthermore, it is necessary in backwashing the deep bed to use large volumes of backwash liquids, thereby presenting a problem of disposal of the oil contaminated backwash liquids themselves.
In a deep granular bed system, flow rates are limited to about ten gallons per minute per square foot of bed area, which is small by comparison to requirements in many applications where apparatus space limitations are imposed. Moreover, solids volume retention capacity of such beds per unit of volume is small, approximately one to three percent of the bed material volume.
Rotary drum separators have also been used for coalescing and separating oil from water. These separators utilize a compressible filter band or matrix of oleophilic hydrophobic material encircling a backing drum and passing continuously beneath a compression roll that squeezes out the oil accumulations in the matrix. Such separators are effective with small size oil droplets such as down to ten microns or smaller, but because of the nature of the self-cleaning action in the machine effected by the squeezing roll, they tend to pass solids along with the oil. In addition, such separators are practically limited to oils of low viscosity. Heavy oils being separated at reasonably high drum rotation speeds tend to greatly shorten the life of the filter band material itself due to repeated rapid compression and release of the band. Under these conditions, the matrix strands are repeatedly overstressed in tension by the direct physical drag of viscous oil being rapidly squeezed from the strands ahead of the squeeze roller. Such separators differ from compressible media separators of the flowthrough type in that the water does not flow unidirectionally but rather flows inwardly through the outer surface regions of the band during injection and then reversely outwardly as the filter band is squeezed against the backing drum.
This invention, in avoiding difficulties and limitations of these other forms of separators, and of prior art coalescing separator devices generally, provides an improved flow-through coalescing separator operable either in a continuous mode with lighter oils, or in a retention mode primarily applicable to mixtures carrying oils of a density near that of water, or to especially dirty mixtures. In the continuous mode, the light oil coalesces in the filter body and is allowed to progressively saturate the same to a limited degree with the water continuing to flow through the body and to leave its downstream face substantially freed of oil and dirt. The filter body medium used is preferably a fully dewindowed or reticulated synthetic foam, such as polypropylene foam, polyurethane foam or similar open-pore visco-elastic material that can be highly compressed to squeeze out accumulated oil and dirt and then reexpanded and that has, or that can be treated to exhibit, an affinity for oil and a repellency to water. In such flow-through separators as in this invention, when operated in the continuous mode, coalesced oil accumulating on the filter body strands eventually builds up to the point of saturation where it sheds off the downstream faces of the strands in the form of coalesced and coalescible large droplets carried downstream by the continued passage of water and that are usually larger than 300 microns in diameter. These emerge into a collection chamber of sufficient volumetric cross-section (i.e., having an outlet preferably near the bottom and small in diameter by comparison) that the low flow rate attending normal flow-through operation of the device allows the oil to rise to the surface according to Stokes' law and be collected in a regulated discharge chamber. Regeneration of the filter body is required only periodically in order to remove solids and heavier oils when they have accumulated to an extent that impedes the flow through the filter medium to an excessive degree for practical rates of production.
In the retention mode, applicable especially to oils near the density of water and to dirty oils, coalescence in the filter media occurs and solid particles accumulate until the medium is substantially saturated with oil and/or solid particles. At that point and before the oil breaks through the filter body to mix with the otherwise purified water passing from the filter body, a condition which may be detected by any of suitable methods, including observation through a window to note oil and/or dirt build-up and/or reduced flow rate, the filter body is regenerated by interrupting the flow-producing pressure and compressing the filter body in order to express the collected oil, solids and water. Since the oil, water and solids are backwashed and squeezed out of the device during such compression, no specific gravity difference is required in order to remove the oil and solids from the mainstream of water which passes through the filter body during the filtration stage. For this reason the device can be used in the retention mode with oils of any specific density.
With a flow-through separator of this general type such as disclosed in the U.S. Pat. Nos. 3,617,551 and 4,022,694, specific flow rates can be much higher than that attainable with granular deep bed separators for example; hence, for a given volumetric process and capacity, the apparatus can be considerably smaller in size. Moreover, solids volume retention is higher, reducing the frequency of regenerating required, and pressure drop across the filter body necessary to obtain effective coalescing filtration can be small (i.e., under five pounds per square inch) thereby reducing or eliminating the aggravation of added emulsification of the oil produced if high-pressure pumping were to be necessary, together with the energy cost of such pumping. This type of apparatus, especially with the improvements afforded by the present invention, can be produced at relatively low cost and will operate durably with virtually negligible maintenance requirements over long periods of time.
Additional patents of some background interest are the following:
______________________________________ U.S. 3,087,708 Great Britain 462,499 3,131,040 3,276,594 Germany 632,698 3,334,042 3,450,632 3,608,727 3,913,513 3,039,441 3,083,778 Re 21,639 ______________________________________
In the prior art proposal of the U.S. Pat. No. 3,617,551, practical difficulties are encountered that limit the effectiveness, reliability and efficiency of the apparatus for useful application in the kinds of environments and for the kinds of uses referred to above. One principal difficulty, for example, lay in the unresolved design dilemna resulting from the necessity of preventing bypass flow around the filter body during flow-through operations by maintaining the filter body in firm contact with the enclosure wall, hence under lateral compression afforded by the restricting effect of the wall. This created a high degree of wall friction resisting compression and reexpansion of the filter body longitudinally of the pressure chamber in order to periodically backwash its pores and squeeze out the oil and dirt. As a result of this high degree of wall friction and the inherently low degree of resilient rebound inherent in the visco-elastic nature of the most suitable filter body materials, expansion of the filter body to its original length and to the full degree of pore-openness was not achieved. Thus, with continued use the openness to flow, and the oil and dirt holding capacity of the filter body, became progressively more limited.
Further, any tendency of the check values to stick closed by the adhesiveness of heavy oils or of tarry components in the U.S. Pat. No. 3,617,551, especially when attempting to operate the system under available low input liquid flow-through producing pressures, such as five pounds or less per square inch, created additional limitations and maintenance requirements. On the other hand, in the U.S. Pat. No. 4,022,694, bidirectional compression and outflow (i.e., out both faces of the filter body) during compression caused the retained contaminants to flow downstream as well as upstream, while solids and heavy residues would increasingly build up with time in the central region of the filter body rather than to be expurgated on each compression cycle.
A further object of the present invention is to overcome such difficulties and limitations in prior flow-through coalescing separator proposals and, more specifically, to provide an improved compressible filter medium flow-through coalescing separator of low-cost construction, using lightweight parts and capable of long-term, trouble-free automatic operation (or manually controlled operation, if preferred), with long operating life of its regeneratable filter body and of its few and simple movable parts.
A further object is to provide such a flow-through oil coalescing separator with low flow-through resistances and relatively high holding capacity of its filter body, both originally and after each regeneration (compression/reexpansion) cycle.
A related object is to provide such a coalescing separator that is capable of accumulating and holding a maximum amount of dirt and oil accumulation, before increased flow-through resistance makes regeneration necessary, and which may be purged of such accumulation quickly by squeezing the filter body preceded by backwashing the same using consistently the same measured amount of backwash water adequate to the job, but not an excessive amount that of itself must be then further processed or separated from the expurgated dirt and oil carried with it.